The greater part of all paper produced contains fillers in the form of mineral material. The original reason for use of fillers is economical as a more expensive fibre material can be replaced by a cheaper mineral material. In addition, many of the paper properties are ameliorated. Above all, improved printability properties are obtained by improving the surface smoothness, opacity and whiteness of the paper. The absorption of printing ink is higher and more even, the picture representation is improved and the paper gloss after calendering can be improved. Other advantages involve better dimensional stability, a better appearance and a better “sensation”.
As mentioned above, the ability of the filler to increase the brightness of the finished paper is important. This results in a reduced or eliminated need to add expensive optical whitening agents. Calcium carbonate is the most frequently used conventional filler but other fillers are also used on a small scale, such as clay, titanium dioxide, talc and silicon dioxide (silica). Also organic fillers are described in the literature. Examples of these are polymers of urea or formaldehyde.
Calcium carbonate exists in several forms when being used as filler and the majority of fine papers contain calcium carbonate up to 25% by weight. Ground marble, ground chalk and precipitated calcium carbonate are the most frequent types. The last mentioned type, precipitated calcium carbonate, is often called PCC, an abbreviation for the English term “Precipitated Calcium Carbonate”. The use of PCC has increased in the last few years and today PCC is the dominant filler at production of fine paper in Sweden.
However, adding fillers causes reduction in the strength of the paper, so there is a practical limit to the amount of fillers normally added to the paper making stock suspension. Another problem connected to a high amount of fillers in the paper is dusting, i.e. the tendency of a paper surface to shed loose and weakly bonded particles, such as PCC originating from fillers, which may accumulate on the printing machinery.